Interpreting carbonate rock data in well logging is a complex task. Spherical pores in these carbonate formations influence certain parameters (e.g., acoustic moduli, resistivity, and permeability) differently than do typical intergranular porosity. The quantification of spherical porosity (e.g., vuggy, oomoldic, etc.) is essential in order to interpret the facies type of the carbonate rock.sup.1.
A technique developed by Brie et al.sup.2 has been used to determine the extent of spherical porosity in carbonates. The present invention seeks to interpret carbonate hydrocarbon formations using a generalization of the Brie et al method. The current invention comprises a method wherein the shear modulus can be used to isolate the amount of spherical porosity in this type of rock, without the requirement of first obtaining prior knowledge of the pore fluid properties.
It is known that compressional slowness of the carbonate rock can be measured in order to qualitatively estimate spherical porosity. However, currently no quantitative estimate can be made in carbonate rock in the presence of a mixture of unknown pore fluids, such as water, gas, oil, etc.
This invention uses the shear modulus to estimate spherical porosity without the need to know the pore fluid composition. The shear modulus is computed from: a) the shear speed measured by an appropriate sonic logging device, such as a Dipole Shear Sonic Imager (DSI); and b) the formation bulk density as measured, for example, by a Litho-Density Tool (LDT).
The invention relies on a mathematical analysis of the shear acoustic response of carbonate rock formations with a mixture of spherical porosity and conventional intergranular porosity. It provides both a forward model for the theoretical response of the rock having a known mixture of the two pore types, and an inverse model that uses the measured response of the rock to evaluate the constituent volumes of each pore type in the total porosity. It is assumed that the total porosity of the formation is known from a combination of nuclear logs such as a Compensated Neutron Log (CNL) and a Litho-Density Log (LDL) and/or the Nuclear Magnetic Resonance (NMR) log. The Dipole Shear Sonic Imager (DSI), Compensated Neutron Log (CNL), Litho-Density Log (LDL), and the Nuclear Magnetic Resonance (NMR) Log are all commercial well logging services provided by Schlumberger Technology Corporation, the assignee of the present invention. These services and the corresponding tools are described, for example, in U.S. Pat. Nos. 5,036,945 to Hoyle et al (DSI), 3,483,376 to Locke et al. (CNL), 3,864,569 to Tittman (LDL), and 5,055,787 to Kleinberg et al. (NMR/CMR).